Electric system for starting and operating a gas discharge lamp

ABSTRACT

A STARTING AND OPERATING CIRCUIT FOR ACROSS THE LINE OPERATION OF A GAS DISCHARGE LAMP IS SHOWN UTILIZING SINGLE SWITCHING MEANS FOR THE DUAL PURPOSE OF DISCHARGING A CAPACITOR TO APPLY AT LEAST ONE STARTING PULSE TO THE DISCHARGE LAMP AND FOR SWITCHING THE LAMP FOR PULSED OPERATION.

United States Patent Inventor Maksymllian A. Michalski Woodslde, N.Y.

Appl. No. 833,534

Filed June 16, 1969 Patented June 28, 1971 Assignce Berkey Photo, Inc.

New York, N.Y.

ELECTRIC SYSTEM FOR STARTING AND OPERATING A GAS DISCHARGE LAMP 8 Claims, 3 Drawing Figs.

US. Cl 315/200, 315/227 Int. Cl HOSb 37/00 Field oiSearch 315/100 Primary Examiner- Roy Lake Assistant Examiner-Lawrence J. Dahl Attorney-Edward T. Connors ABSTRACT: A starting and operating circuit for across the line operation of a gas discharge lamp is shown utilizing single switching means for the dual purpose of discharging a capacitor to apply at least one starting pulse to the discharge lamp and for switching the lamp for pulsed operation.

PATENTEMuuzsmn 35 ,5959

V J E .vrunx-ug ATTORNEY ELECTRIC SYSTEM FOR STARTING AND OPERATING A GAS DISCHARGE LAMP The present invention relates to an electric system including an electric discharge lamp for operation directly connected in series with switching means across a source of alternating current, and to a method of operation thereof.

Electric systems in accordance with the invention are used for copyboard illumination systems, and for back or transparency illumination systems, both used in conjunction with graphic arts cameras. The systems may be used also for black and white and color enlarger light sources, both condenser and diffused light types. In addition the systems may be used for exposure of presensitized reproduction materials such as are used for contact printing, or for use with vacuum printing frames as usedin photographic and graphic arts.

Heretofore, electric systems have been provided for operation in the pulsed mode for producing a plurality of successive flashes of light in which the systems have included energy storage devices such as inductors and capacitors connected in series or in parallel with the gas discharge lamps. It was believed that these elements were necessary to store the electrical energy to provide high instantaneous loading of the discharge lamps to achieve an efficient conversion of electri cal energy to light energy in the form of high intensity flashes. The elements of such systems were expensive, bulky, and sometimes prone to failure under conditions of repetitive operation.

In a copending application Ser. No. 751,403, filed Aug. 9, 1968 by the applicant herein, there is shown an electric system including a discharge lamp directly connected across an alternating current line, the discharge lamp having an operating voltage higher than the line voltage so that the lamp is extinguished at the end of each half cycle and reignited during the next half cycle.

In another copending application Ser. No. 817,912 filed Apr. 21, 1969 by the applicant herein there is shown an electric system including a discharge lamp directly connected across an alternating current line subject to variations between upper and lower voltages, the lamp having an operating voltage at the lower voltage, phase angle control of the switching means being provided to switch the lamp later in each half cycle for voltages higher than its rated voltage so as to maintain the wattage loading of the lamp at a desired level.

In the operation of discharge lamp systems utilizing separate starting circuits including separate switching devices such as in the applications referred to above care must be taken to make sure that the starting circuit is synchronized with the operating circuit or supply line so as to provide inphase operation during the starting or ignition cycle of the lamps. It has been found that if the starting circuits are out-of-phase with the operating circuits the lamps may not ignite at all, or the lamps may ignite earlier inphase, resulting in a much higher power loading of the lamps than desired with consequent danger of damage to the lamp switching means, and other components. This out-of-phase condition may also occur under a condition of extremely high input voltage when under proper operation the phase control should delay the pulses in the cycle so as to achieve normal operating lamp wattage.

Another difficulty with an out-of-phase condition between the starting and operating circuits is difficulty in starting the lamps. Furthermore, with a random firing starting signal there is no controlover the light intensity and power loading of the lamps during the starting signal.

The present invention aims to provide improved operation with reduced flickering during the starting period of discharge lamps operated directly across the line by providing synchronized control between the starting and operating pulses.

In accordance with the invention a starting and operating circuit is provided for a gas discharge lamp in which the circuit utilizes single switching means for discharging a capacitor through starting means and for connecting the source to the lamp so that starting and operating pulses are synchronized.

The system, in accordance with the invention, is also advantageous in that fewer components are required than for prior systems, and the time spent in alignment ofthe circuits is greatly reduced.

Other objects and advantages of the invention will be apparent from the following description and from the accompanying drawings which show, by way of example, an embodiment of the invention.

In the drawings:

FIG. 1 is a schematic diagram of an electrical system in accordance with the invention and in which the starting pulse capacitor is connected in parallel with the switching means.

FIG. 2 is a schematic diagram of another embodiment of the invention in which the starting pulse capacitor is connected in series with the switching means.

FIG. 3 shows a series of waveforms illustrating the operation of the electric system in accordance with the invention.

Referring to the drawings there is shown in FIG. I a schematic diagram of an electric system in accordance with the invention. This system includes a pair of terminals 1 and 2 adapted to be connected to a source of alternating current which may be of the order of l.90250 volts. Across the terminals l and 2 may be connected a bypass capacitor C1, for the purpose of completing the circuit for high frequency starting pulses for the ignition of discharge lamp VI. In some installations the bypass capacitor may be omitted, the high frequency starting pulses being passed through resistor R1 and capacitor C2 provided the resistor R1 is oflow value.

The discharge lamp V1 is connected in series with a secondary of a starting transformer T1 and a solid-state switching device TRl across the terminals 1 and 2. Although shown with its secondary series connected in the circuit, the starting transformer T1 may be connected in any conventional manner, as by the use of a third electrode as in FIG. 2, or capacitively coupled as known in the art. The solid-state switching device TRI may be any suitable device such as a biswitch, a triac, a switching diode, or a pair of controlled rectifiers, all of which are adapted to be controlled by means known in the art.

A suitable control means for operating the system in accordance with the invention using a triac as the switching means includes a diac DI connected to the gate of TRI. Control means for the diac Dl includes a resistor R2 in series with a variable resistor R3 and a capacitor C3 connected across the triac TR]. A resistor R4 is connected between thejunction of the variable resistor R3 and the capacitor C3 to the gate of the diac CI, a filtering capacitor C4 being connected from the diac end of the resistor R4 to the terminal 2. The operation of this circuit is well known in the art and is described in General Electric, SCR Manual, 4th edition, Chapter 9, pages 173 through 189 and in particular figure 9.l5 on page 189, subchaptcr 9.4.2.

Triggering means for igniting the discharge lamp V1 includes a primary P for the transformer T1 connected to the junction between capacitor C2 and resistor R1 connected across the terminals 1 and 2. A switch S1 is connected in series with the primary P.

A bypass impedance R5 may be connected across the solidstate device TRI. The impedance R5 is termed a keep alive impedance and is included to maintain ionization in the lamps of higher pressure. For example, no impedance is required in the circuit of FIG. 1 for a discharge lamp of 20 to 35 millimeter pressure. A 40 millimeter lamp requires about a 500 ohm impedance while a millimeter lamp might require about a 200 ohm impedance. The impedance R5 is preferably in the form of a resistance although in some cases a choke, or a capacitor in series with a resistor, may be substituted for the resistance. The impedance R5 provides a path for ionization current for the discharge lamp during the period in which the solid-state device TRl is not conducting. In the circuit of FIG. 2 the impedance R5 also provides a path for charging current for the capacitor C5.

The gas discharge lamp VI is ignited by closing the switch S1 for 2 or 3 seconds. The switch S1 may be a time delay switch ordinarily closed for a period of 2 or 3 seconds. The triggering capacitor C2 is discharged through the primary P of the starting transformer TI. As determined by the selective values of the resistor RI and of the capacitor C2, one or more pulses will be produced through the starting transformer T1. When the voltage across the triac TRll reaches a preset triggering value the switching device TRl is turned on discharging the capacitor C2 into the primary of the transformer T1 resulting in both the high frequency voltage and the operating voltage appearing across the lamp Vl simultaneously so that the lamp V1 becomes ionized and conducting. The high frequency starting pulse circuit is completed through the capacitor CI in the event it is used. The capacitor C] also acts as a radio frequency interference filter in blocking any possible radiation from the high frequency starting pulses.

The values of RI and C2 are so selected as to produce a satisfactory starting pulse without passing excessive power through the switching device TRI. lfthe value ofRl is too low excessive current will flow which is not desirable as the light level during the starting period will be much higher than desired and the size of the components necessarily must be larger, if the value of R1 is too high the current will be too low making the lamp difficult to ignite.

In the circuit shown in FIG. I with a resistor R1 of I50 ohms and a capacitor C1 of 2 mfd., the number of pulses per half cycle is 4. During the period while the lamp is being pulsed before becoming ignited the pulses act to heat up the electrodes of the lamp and to ionize its gas. The impedance R] is used for current limiting purposes. In the event an inductive reactance is used in place of the limiting resistance Rl it is practical to operate the starting means continuously without use of the time delay switch S1 because the current through the switching means TRI is less than with a resistor and thus the wattage loss is small.

Referring to FIG. 3, waveforms A, B and C have been taken from the circuit of FIG. I. Waveform A illustrates the voltage across the capacitor C2 before the lamp V] has become ignited and is in conduction. It will be noted that 4 pulses are clearly visible. Waveform B illustrates the voltage across the capacitor C2 after the lamp V1 has become ignited. It will be noted that the lamp becomes conductive upon the first pulse thus discharging the capacitor C2. Waveform C is a single starting pulse current waveform. The starting pulse is a typical damped wave oscillation of approximately 30 kHz. frequency, the peak current being about 60 amperes, and the single pulse duration being approximately 100 microseconds.

In FIG, 2 there is shown another embodiment of the invention in which corresponding components aredesignated by the same indicia as in FIG. I where similar. The circuit of FIG. 2 differs from the circuit shown in FIG. 1 in that the starting pulse capacitor C5 of FIG. 2 is connected in series with the switching device TRl. As stated before, in FIG. I the capacitor C2 series circuit is connected in parallel with the switching device TRI. The advantage of the circuit of FIG. 2 is that the capacitor C5 is discharged in series with the switching device TRI thereby eliminating the resistor R1 which must be large and thus dissipates a large amount of wattage.

The capacitor C5 has about times the capacitance of the capacitor C2 of FIG. I and the resistor R6 is about one twohundredths of the resistance ofthe resistor R1 of FIG. 2. In the circuit of FIG. 2 the resistor R6 is of 10 watt capacity while the resistor RI of FIG. I is of watt or higher wattage.

The function of the resistor R6 is to compensate for phase shift and to limit the peak pulse current through the switching device TRI. It will be noted that the voltage waveform E of FIG. 3 differs from the corresponding waveform A in that the waveform E has only two pulses per half cycle. However, by varying the values ofcomponents the number of pulses may be varied as desired.

Waveform E shows the voltage across the capacitor C5 during ignition. Waveform D shows the current through the starting circuit after the light is ignited. The current waveform D shows a peak current of about 80 amperes, the single pulse duration being approximately 150 microseconds.

Capacitor CI 2 mfd. Capacitor C2 2 mfd. Capacitor C3 0.05 mfd. Capacitor C4 ().l mfd. Capacitor C5 20 mfd. Resistor RI I50 ohms Resistor R2 45 K ohms Resistor R3 56 K ohms Resistor R4 7.5 K ohms Resistor R5 500 ohms Resistor R6 0.8 ohm PI Primary 3 turns secondary turns ferrite core one-half square inch in area TRI RCA 2N5442 DI RCA IN54I I SI Time delay 3 sec.

While the invention has been described and illustrated with reference to specific embodiments thereof it will be understood that other embodiments may be resorted to without departing from the invention. Therefore, the form of the invention set out above should be considered as illustrative and not as limiting the scope of the following claims.

Iclaim:

I. A continuously pulsing electric system for starting and operating a gas discharge lamp directly from an alternating current source, the electric system comprising a gas discharge lamp, starting means for said lamp including a starting pulse capacitor, means to charge said capacito'r from said source, means coupling said capacitor to said starting means, solidstate switching means connected in series with said lamp across said source, said starting means connected to the load side of said switching means so that said switching means is effective to discharge said starting pulse capacitor into said starting means simultaneously with energization of said gas discharge lamp, and phase control means connected across said switching means and including an output connected to said switching means to switch said solidstate switching means so that at a desired phase position of the operating source frequency at least one starting pulse and an operating pulse are synchronously applied to said discharge lamp.

2. An electric system according to claim I in which a bypass capacitor is connected across said source.

3. An electric system in accordance with claim 1 in which said starting means includes a starting transformer having a primary and a secondary winding connected in starting relationship to said lamp, and said phase control means includes a phase control capacitor, means to charge said phase control capacitor from said source, and means to discharge said phase control capacitor to switch said switching means.

4. An electric system in accordance with claim 3 in which said starting pulse capacitor is connected across said source in series with a current limiting impedance, and said primary winding of said starting transformer is connected in parallel with said starting pulse capacitor by said switching means.

5. An electric system in accordance with claim 3 in which said starting pulse capacitor is connected across said source in series with said primary winding of said starting transformer and a current limiting impedance in series by said switching means.

series with said primary winding of said starting transformer and a current limiting impedance by said switching means, and an impedance is connected across said switching means.

8. An electric system in accordance with claim 1 in which an impedance is connected in parallel with said switching means. 

